View source for Earth:Deposition (geology)

{{short description|Geological process in which sediments, soil and rocks are added to a landform or landmass}}
[[File:deposition erosion.jpg|thumb|right|alt=Map of Cape Cod showing shores undergoing erosion (cliffed sections) and shores characterized by marine deposition (barriers).|Map of Cape Cod showing shores undergoing erosion (cliffed sections) in yellow, and shores characterized by marine deposition (barriers) in blue.<ref>{{cite journal|last1=Oldale|first1=Robert N.|title=Coastal Erosion on Cape Cod: Some Questions and Answers|journal=Cape Naturalist, the Journal of the Cape Cod Museum of Natural History|date=1999|volume=25|pages=70–76|url=https://woodshole.er.usgs.gov/staffpages/boldale/capecod/index.html|access-date=15 October 2016|url-status=dead|archive-url=https://web.archive.org/web/20160315040145/https://woodshole.er.usgs.gov/staffpages/boldale/capecod/index.html|archive-date=2016-03-15}}</ref>]]

'''Deposition''' is the geological process in which sediments, [[Chemistry:Soil|soil]] and [[Earth:Rock (geology)|rock]]s are added to a [[Earth:Landform|landform]] or [[Earth:Landmass|landmass]]. Wind, ice, water, and [[Physics:Gravity|gravity]] transport previously [[Earth:Weathering|weathered]] surface material, which, at the loss of enough [[Physics:Kinetic energy|kinetic energy]] in the fluid, is deposited, building up layers of sediment.

This occurs when the forces responsible for sediment transportation are no longer sufficient to overcome the forces of gravity and [[Physics:Friction|friction]], creating a resistance to motion; this is known as the null-point hypothesis. Deposition can also refer to the buildup of sediment from [[Chemistry:Organic matter|organically derived matter]] or [[Chemistry:Chemical process|chemical process]]es. For example, [[Earth:Chalk|chalk]] is made up partly of the microscopic [[Chemistry:Calcium carbonate|calcium carbonate]] skeletons of marine [[Earth:Plankton|plankton]], the deposition of which induced chemical processes ([[Earth:Diagenesis|diagenesis]]) to deposit further calcium carbonate. Similarly, the formation of [[Earth:Coal|coal]] begins with the deposition of organic material, mainly from plants, in [[Earth:Hypoxia (environmental)|anaerobic]] conditions.

==Null-point hypothesis==

The null-point hypothesis explains how [[Earth:Sediment|sediment]] is deposited throughout a shore profile according to its grain size. This is due to the influence of hydraulic energy, resulting in a seaward-fining of sediment particle size, or where fluid forcing equals gravity for each grain size.<ref name="Jolliffe">{{cite journal | first1=I. P. | last1=Jolliffe | title=Littoral and offshore sediment transport | journal=Progress in Physical Geography | year=1978 | volume=2 | issue=2 | pages=264–308 | issn=0309-1333 | doi=10.1177/030913337800200204| s2cid=128679961 }}</ref> The concept can also be explained as "sediment of a particular size may move across the profile to a position where it is in equilibrium with the wave and flows acting on that sediment grain".<ref name="Horn">{{cite journal | first1=Diane P | last1=Horn | title=A review and experimental assessment of equilibrium grain size and the ideal wave-graded profile | journal=Marine Geology | year=1992 | volume=108 | issue=2 | pages=161–174 | issn=0025-3227 | doi=10.1016/0025-3227(92)90170-M| bibcode=1992MGeol.108..161H }}</ref> This sorting mechanism combines the influence of the down-slope gravitational force of the profile and forces due to flow asymmetry; the position where there is zero net transport is known as the null point and was first proposed by Cornaglia in 1889.<ref name="Horn"/> Figure 1 illustrates this relationship between sediment grain size and the depth of the marine environment.
[[File:null point theory image1.png|thumb|right|Figure 1. Illustrating the sediment size distribution over a shoreline profile, where finer sediments are transported away from high energy environments and settle out of suspension, or deposit in calmer environments. Coarse sediments are maintained in the upper shoreline profile and are sorted by the wave-generated hydraulic regime]]

The first principle underlying the null point theory is due to the gravitational force; finer sediments remain in the water column for longer durations allowing transportation outside the surf zone to deposit under calmer conditions. The gravitational effect or settling velocity determines the location of deposition for finer sediments, whereas a grain's internal angle of friction determines the deposition of larger grains on a shore profile.<ref name=" Horn"/> The secondary principle to the creation of seaward sediment fining is known as the hypothesis of asymmetrical thresholds under waves; this describes the interaction between the oscillatory flow of waves and tides flowing over the wave ripple bedforms in an asymmetric pattern.<ref name="Masselink">{{cite book | chapter=Chapter Five—Sediments, boundary layers and transport: Coastal processes and geomorphology | chapter-url=https://books.google.com/books?id=v5XsAgAAQBAJ&pg=PA105 | first1=Gerd | title=Introduction to Coastal Processes and Geomorphology | publisher=Hodder Education | last2=Hughes | first2=Michael | last3=Knight | first3=Jasper | last1=Masselink | year=2011 | edition=2nd | location=London | pages=105–147 | isbn=978-1-4441-2241-1 | oclc=795119869}}</ref> "The relatively strong onshore stroke of the waveforms an eddy or vortex on the lee side of the ripple, provided the onshore flow persists, this eddy remains trapped in the lee of the ripple. When the flow reverses, the eddy is thrown upwards off the bottom and a small cloud of suspended sediment generated by the eddy is ejected into the water column above the ripple, the sediment cloud is then moved seaward by the offshore stroke of the wave."<ref name="Masselink"/> Where there is symmetry in ripple shape the vortex is neutralised, the eddy and its associated sediment cloud develops on both sides of the ripple.<ref name="Masselink"/> This creates a cloudy water column which travels under the tidal influence as the wave orbital motion is in equilibrium.

The Null-point hypothesis has been quantitatively proven in [[Place:Akaroa|Akaroa]] Harbour, New Zealand, The Wash, U.K., Bohai Bay and West Huang Sera, Mainland China, and in numerous other studies; Ippen and Eagleson (1955), Eagleson and Dean (1959, 1961) and Miller and Zeigler (1958, 1964).

=== Deposition of non-cohesive sediments ===

Large-grain sediments transported by either bedload or suspended load will come to rest when there is insufficient bed shear stress and fluid turbulence to keep the sediment moving;<ref name=" Masselink"/> with the suspended load this can be some distance as the particles need to fall through the water column. This is determined by the grain's downward acting weight force being matched by a combined buoyancy and fluid drag force<ref name="Masselink"/> and can be expressed by:

:<math> \frac{4}{3} \pi R^3 \rho_s g = \frac{4}{3}  \pi R^3 \rho g  + \frac{1}{2} \Complex_d \rho \pi R^2 w_s^2\,</math>
Downward acting weight force	=  Upward-acting buoyancy force  +  Upward-acting fluid drag force<ref name="Masselink"/>

where:
*''&pi;'' is the ratio of a circle's circumference to its diameter.
*''R'' is the radius of the spherical object (in m),
*''&rho;'' is the mass density of the fluid (kg/m<sup>3</sup>),
*''g'' is the gravitational acceleration (m/s<sup>2</sup>),
*''C<sub>d</sub>'' is the drag coefficient, and
*''w<sub>s</sub>'' is the particle's settling velocity (in m/s).

In order to calculate the drag coefficient, the grain's [[Reynolds number]] needs to be discovered, which is based on the type of fluid through which the sediment particle is flowing, laminar flow, turbulent flow or a hybrid of both. When the fluid becomes more viscous due to smaller grain sizes or larger settling velocities, the prediction is less straightforward and it is applicable to incorporate Stokes Law (also known as the frictional force, or drag force) of settling.<ref name="Masselink"/>

=== Deposition of cohesive sediments ===

The cohesion of sediment occurs with the small grain sizes associated with silts and clays, or particles smaller than 4ϕ on the phi scale.<ref name=" Masselink"/> If these fine particles remain dispersed in the water column, [[Physics:Stokes' law|Stokes law]] applies to the settling velocity of the individual grains,<ref name="Masselink"/> although due to seawater being a strong [[Physics:Electrolyte|electrolyte]] bonding agent, [[Chemistry:Flocculation|flocculation]] occurs where individual particles create an electrical bond adhering each other together to form flocs.<ref name="Masselink"/> "The face of a clay platelet has a slight negative charge where the edge has a slight positive charge when two platelets come into close proximity with each other the face of one particle and the edge of the other are electrostatically attracted."<ref name="Masselink"/> Flocs then have a higher combined mass which leads to quicker deposition through a higher fall velocity, and deposition in a more shoreward direction than they would have as the individual fine grains of clay or silt.

=== The occurrence of null point theory ===

Akaroa Harbour is located on [[Earth:Banks Peninsula|Banks Peninsula]], Canterbury, New Zealand, {{Coord|display=inline|43|48|S|172|56|E}}. The formation of this harbour has occurred due to active erosional processes on an extinct shield volcano, whereby the sea has flooded the caldera, creating an inlet 16&nbsp;km in length, with an average width of 2&nbsp;km and a depth of −13 m relative to mean sea level at the 9&nbsp;km point down the transect of the central axis.<ref name="Hart">{{cite report|publisher=University of Canterbury and DTec Consulting Ltd.|first1=Deirdre E.|last1=Hart|url=http://ecan.govt.nz/publications/Reports/upper-akaroa-harbour-seabed-bathymetry-soft-sediments-baseline-mapping.pdf|title=Upper Akaroa Harbour Seabed Bathymetry and Soft Sediments: A Baseline Mapping Study|last2=Todd|first2=Derek J.|last3=Nation|first3=Thomas E.|last4=McWilliams|first4=Zara A.|series=Coastal Research Report 1|year=2009|isbn=978-1-86937-976-6| id=ECan Report 09/44|access-date=2016-05-31|archive-url=https://web.archive.org/web/20160201152154/http://ecan.govt.nz/publications/Reports/upper-akaroa-harbour-seabed-bathymetry-soft-sediments-baseline-mapping.pdf|archive-date=2016-02-01|url-status=dead}}</ref> The predominant storm wave energy has unlimited fetch for the outer harbour from a southerly direction, with a calmer environment within the inner harbour, though localised harbour breezes create surface currents and chop influencing the marine sedimentation processes.<ref name="HeuffSpigel2005">{{cite journal|last1=Heuff|first1=Darlene N.|last2=Spigel|first2=Robert H.|last3=Ross|first3=Alex H.|title=Evidence of a significant wind‐driven circulation in Akaroa Harbour. Part 1: Data obtained during the September‐November, 1998 field survey|journal=New Zealand Journal of Marine and Freshwater Research|volume=39|issue=5|year=2005|pages=1097–1109|issn=0028-8330|doi=10.1080/00288330.2005.9517378|doi-access=free}}</ref> Deposits of loess from subsequent glacial periods have in filled volcanic fissures over millennia,<ref name="Raeside1964">{{cite journal | first1=J. D. | last1=Raeside | title=Loess Deposits of the South Island, New Zealand, and Soils Formed on them | journal=New Zealand Journal of Geology and Geophysics | year=1964 | volume=7 | issue=4 | pages=811–838 | issn=0028-8306 | doi=10.1080/00288306.1964.10428132| doi-access=free }}</ref> resulting in volcanic basalt and loess as the main sediment types available for deposition in Akaroa Harbour

[[File: Akaroa Harbour jpeg.png|thumb|Figure 2. Map of Akaroa Harbour showing a fining of sediments with increased bathymetry toward the central axis of the harbour. Taken from Hart et al. (2009) and the University of Canterbury under the contract of Environment Canterbury.<ref name="Hart"/>]]

Hart et al. (2009)<ref name="Hart"/> discovered through bathymetric survey, [[Engineering:Sieve|sieve]] and [[Engineering:Pipette|pipette]] analysis of subtidal sediments, that sediment textures were related to three main factors: depth, distance from shoreline, and distance along the central axis of the harbour. This resulted in the fining of sediment textures with increasing depth and towards the central axis of the harbour, or if classified into grain class sizes, "the plotted transect for the central axis goes from silty sands in the intertidal zone to sandy silts in the inner nearshore, to silts in the outer reaches of the bays to mud at depths of 6 m or more".<ref name=" Hart"/> See figure 2 for detail.

Other studies have shown this process of the winnowing of sediment grain size from the effect of hydrodynamic forcing; Wang, Collins and Zhu (1988)<ref name=" Wang">{{cite conference | first1=Y. | title=Proceedings of the International Symposium on the Coastal Zone | last1=Wang | chapter=A comparative study of open coast tidal flats: The Wash (U.K.), Bohai Bay and West Huang Sera (Mainland China) | publisher=China Ocean Press | last2=Collins | first2=M.B. | last3=Zhu | first3=D. | year=1988 | location=Beijing | pages=120–134}}</ref> qualitatively correlated increasing intensity of fluid forcing with increasing grain size. "This correlation was demonstrated at the low energy clayey tidal flats of Bohai Bay (China), the moderate environment of the Jiangsu coast (China) where the bottom material is silty, and the sandy flats of the high energy coast of The Wash (U.K.)." This research shows conclusive evidence for the null point theory existing on tidal flats with differing hydrodynamic energy levels and also on flats that are both erosional and accretional.

Kirby R. (2002)<ref name="Kirby">{{cite book | chapter=Distinguishing accretion from erosion-dominated muddy coasts | editor-first1=T. | editor-first2=Y. | editor-first3=J.-A. | editor-last1=Healy | editor-last2=Wang | editor-last3=Healy | chapter-url=https://books.google.com/books?id=hXKKr5O2C1wC&pg=PA61 | title=Muddy Coasts of the World: Processes, Deposits and Function | publisher=Elsevier | last=Kirby | first=R. | year=2002 | pages=61–81 | isbn=978-0-08-053707-8}}</ref> takes this concept further explaining that the fines are suspended and reworked aerially offshore leaving behind lag deposits of the main bivalve and gastropod shells separated out from the finer substrate beneath, waves and currents then heap these deposits to form [[Earth:Chenier|chenier]] ridges throughout the tidal zone, which tend to be forced up the foreshore profile but also along the foreshore. Cheniers can be found at any level on the foreshore and predominantly characterise an erosion-dominated regime.<ref name="Kirby"/>

=== Applications for coastal planning and management ===

The null point theory has been controversial in its acceptance into mainstream coastal science as the theory operates in [[Physics:Dynamic equilibrium|dynamic equilibrium]] or unstable equilibrium, and many fields and laboratory observations have failed to replicate the state of a null point at each grain size throughout the profile.<ref name=" Horn"/> The interaction of variables and processes over time within the environmental context causes issues; "a large number of variables, the complexity of the processes, and the difficulty in observation, all place serious obstacles in the way of systematisation, therefore in certain narrow fields the basic physical theory may be sound and reliable but the gaps are large"<ref name="Russell">{{cite journal | title=Coast Erosion and Defence: Nine Questions and Answers | last=Russell | first=R.C.H. | journal=Hydraulics Research Paper | year=1960 | volume=3 }}</ref>

Geomorphologists, engineers, governments and planners should be aware of the processes and outcomes involved with the null point hypothesis when performing tasks such as [[Earth:Beach nourishment|beach nourishment]], issuing building consents or building [[Earth:Coastal management|coastal defence]] structures. This is because sediment grain size analysis throughout a profile allows inference into the erosion or accretion rates possible if shore dynamics are modified. Planners and managers should also be aware that the coastal environment is dynamic and contextual science should be evaluated before the implementation of any shore profile modification. Thus theoretical studies, laboratory experiments, numerical and hydraulic modelling seek to answer questions pertaining to littoral drift and sediment deposition, the results should not be viewed in isolation and a substantial body of purely qualitative observational data should supplement any planning or management decision.<ref name=" Jolliffe"/>

==See also==
*{{annotated link|Earth:Cementation (geology)}}
*{{annotated link|Earth:Cross-bedding}}
*{{annotated link|Earth:Drift (geology)}}
*{{annotated link|Chemistry:Flocculation}}
*{{annotated link|Earth:Longshore drift}}
*{{annotated link|Earth:Overbank}}
*{{annotated link|Earth:Sedimentary rock}}
*{{annotated link|Earth:Sedimentary structures}}
*{{annotated link|Chemistry:Settling}}
*{{annotated link|Shields parameter}}
*[[Earth:Sorting (sediment)|Sorting]] - sediment sorting 
*{{annotated link|Physics:Stokes' law}}
*{{annotated link|Earth:Superficial deposits}}

==References==
{{Reflist|30em}}

{{River morphology}}
{{Rivers, streams and springs}}

[[Category:Deposition (geology)| ]]
[[Category:Sedimentology]]

{{Sourceattribution|Deposition (geology)}}